Estrogen receptors in the rat uterus. Retention of hormone-receptor complexes

Abstract

The retention pattern and biochemical characteristics of estrogen receptors in the nuclei of uterine cells were studied as a function of time after the in vivo injection of estradiol (E2) to immature female rats. One h after the injection of 0.1 .mu.g of tritiated E2, approximately 0.20 pmol/uterus of receptor bound hormone is retained in uterine nuclei. This dose of E2 produces a maximal uterotrophic response. Six h after E2 administration, uterine nuclei retain 0.04-0.08 pmol of hormone/uterus. Hormone receptor complexes extracted from uterine nuclei 1, 3 and 6 h after in vivo injection of hormone have similar structural and binding characteristics. Receptors extracted at all 3 times sediment at 5S in high salt gradients and have a dissociation binding constant of approximately 3 nM for E2. The wash-out curves of receptors as a function of salt concentration are identical for uterine nuclei from animals treated for 1 or 6 h with E2, suggesting that the nature of the nuclear binding of receptors is not altered during this time interval. Experiments utilizing the injection of unlabeled E2, followed by an in vitro exchange procedure with tritiated E2, indicated that the total nuclear estrogen receptor sites, i.e., filled and vacant, decreased similarly.